Remember X-Men: Battle Of The Atom? Of course you do. There was a first issue that focused on the All-New X-Men, colloquially known as the O5, standing for the Original Five X-Men. And what happened when the young Scott Summers was attacked. And then healed.

Before the healing, the older Scott Summers disappeared.

Establishing that the O5 were part of the same timeline as the current X-Men.